European Published Patent Application No. 0 955 524 describes a flow sensor for ascertaining the air mass drawn in by an internal combustion engine having two heating resistors and two reference temperature sensors. With the aid of the reference temperature sensors, the temperature of the air flowing past the flow sensor is measured before the air flows across the heating resistors. The two heating resistors are used for measuring the air mass flowing across the flow sensor. For this purpose, one makes use of the effect that the first heating resistor in the direction of flow heats the air flowing across it and that as a result the second heating resistor in the direction of flow requires less heating energy than the first heating resistor in order to achieve a specified temperature. The cooling of the first heating resistor in the direction of flow results in a reduced electrical resistance of this heating resistor, while the second heating resistor is cooled only a little by the air heated by the first heating resistor such that the electrical resistance of the second heating resistor, all other initial conditions being equal, is greater than that of the first heating resistor. The air mass flowing past the flow sensor is inferred from the difference between the temperature-dependent resistances of the first heating resistor and of the second heating resistor or from the difference between the heating voltages required to maintain constant (excess) temperatures.
It is further known to assign to each heating resistor a temperature sensor of its own for measuring the temperature of the heating resistor.
Hence in this circuit there are two heating resistors, two temperature sensors and two reference temperature sensors having altogether 2×6=12 terminals. If these 12 terminals are to be accommodated on a chip, then the size of the chip will be predetermined by, among other things, the number of terminals. The costs of a chip depend, among other things, also on its size.
German Published Patent Application No. 103 24 292 describes a measuring device having two heating resistors, two temperature sensors and one reference temperature sensor having altogether six terminals. In this measuring device, both temperature sensors are used to ascertain the air mass flowing across the measuring device.